1. Field of the Invention
The present invention relates to a scanning optical device adapted for use in a recording apparatus such as a laser beam printer, and more particularly to a scanning optical device incorporating a rotary polygon mirror and a motor for rotating the same, in an enclosed casing.
2. Related Background Art
In a scanning optical device employed in an image recording apparatus such as a laser beam printer or a laser copying machine, a photosensitive member is scanned with a light beam deflected by a deflecting device to form an electrostatic latent image, which is then rendered visible as a toner image, in a developing device. The toner image is transferred onto a recording sheet, and is fixed thereto by heating in a fixing device to obtain a print.
FIG. 1 shows a conventional scanning optical device incorporating a rotary polygon mirror and a motor for driving the same. In an outer casing 101 there is fixed a base plate 102, and a bearing 103 fixed thereon which rotatably supports a shaft 104, which supports a rotor 106 by means of a flange 105. A rotary polygon mirror 107 is urged toward the flange 105 by a press spring 108. The rotor 106 is positioned opposed to a stator 109 fixed to the base plate 102, whereby the rotor 106 and the stator 109 constitute a motor M.sup.0 A driving circuit 110 for driving the motor M.sub.0 is composed of components 110a-110c mounted on a wiring board 111 extending from under the stator 109 to the side of the base plate 102, and is connected to an unrepresented control circuit which controls the revolution of the motor M.sub.0 with a high precision. The rotary polygon mirror 107 rotated by the motor M.sub.0 deflects a laser beam emitted from a laser unit 112 shown in FIG. 2, thereby scanning a photosensitive member (not shown). An opening, positioned in the upper part in the illustration, of the outer casing 101 is closed by a lid 113, and the gap between the edges of the lid 113 and the opening of the outer casing is sealed by a gasket 114.
In such conventional device, however, since the sealed outer casing incorporates the motor for driving the rotary polygon mirror and the driving circuit therefor, the internal temperature in the casing is significantly elevated by the heat generation of these components, thus leading to the following drawbacks:
(1) The viscosity of the lubricant of the motor bearing is varied, thus leading to unstable rotation of the shaft. As a result, the shaft generates vibration, which induces significant noises and deterioration in performance; PA1 (2) As the electric characteristics of the driving circuit vary with the temperature, the motor cannot be controlled with a high precision.